In previous apparatus of this kind the instrument has been carried by a threaded stem which has been sealed to the conduit by means including a gland. The use of the threaded stem prevents the instrument from being ejected from the conduit in the event of faulty use and is therefore "fail safe". The gland, however, has been such as to provide an easy path for the extrusion of the gland material and has therefore been subject to leakage.
Use has also been made previously of a separate screw jack to push the instrument into the conduit. This deals with the leakage problem but has involved the use of a collet to grip the instrument shaft and this does not provide a "fail safe" arrangement as there can easily be slippage between the collet and the instrument shaft if there is misuse.